Brazing is a metal-joining process in which two or more metal articles are joined together by melting and flowing a braze material, which may be a metal or a metal alloy, into a joint that is defined between the metal articles. More specifically, brazing is a thermally induced metallic bonding process that occurs below the melting point of the metals to be joined with the introduction between them in the joint of the braze material, which has a lower melting point than the metals to be joined. Upon subsequent cooling, the braze material forms a fillet that bonds the metal articles together at faying surfaces thereof. For assured selective melting of only the braze material during heating, the melting point of the braze material is typically chosen to be at least about 30° to 40° C. lower than that of the metal in the metal articles to be joined. For brazing aluminum articles together, for example, a suitable brazing alloy is an Al—Si eutectic composition, which melts at about 577° C.
The brazing process involves a number of metallurgical and chemical processes that take place both on the surface and within the materials. For example, good wetting and spreading of the molten braze material on the surface of the metal articles determine whether capillary action will occur. Capillary flow is the dominant physical principle that ensures an acceptable braze fillet in a properly spaced joint, provided molten braze material wets both surfaces that are to be joined. Capillary flow is affected by the presence of oxide films, surface roughness and the condition and properties of the brazing atmosphere.
Various techniques are employed to apply the braze material to the metal articles to be brazed. In one such technique, at least one of the surfaces being joined is pre-clad with a layer of aluminum brazing alloy. Such pre-clad articles, generally known as brazing sheet, are relatively costly, and in many instances it is preferred to provide the braze material in some form other than cladding. One known alternative is to apply the braze material to or adjacent to one or both joining surfaces in powdered or particulate form carried in a suitable liquid or paste-like vehicle. In such methods, a mixture of the braze material in powdered form, in an aqueous carrier or mixed with a binder, is coated on the surfaces to be joined. When included in an aqueous carrier, the coating is then dried and the surfaces are then heated to a brazing temperature whereby the brazing is completed. When included with a binder, e.g., a polymeric material that binds the braze material to the surfaces of the article to be brazed, the binder is generally burned off prior to brazing through pre-heating of the article after deposition of the coating thereon.
Brazing of some metal articles, such as aluminum and its alloys, is particularly difficult because an oxide film forms on the surface when exposed to air. The barrier action of the oxide film on aluminum hinders wetting and inhibits capillary flow. To enable intimate contact between the molten braze material and the base metal of the article, it is necessary to disrupt the oxide, for example through the use of an inorganic salt that acts as a flux. An inert brazing atmosphere free from oxygen and water vapor may be facilitated to prevent re-oxidation of the molten braze material and oxidation of the flux itself. This may be achieved by brazing under nitrogen or by using a vacuum. The flux must be capable of disrupting and/or otherwise remove the metal oxides at the brazing temperatures while remaining essentially inert with respect to the metal of the article, e.g., aluminum, at the brazing temperatures. Since fluxes are usually reactive only when at least partially molten, fluxes for aluminum brazing, for example, should as a practical matter be partly or wholly molten at brazing temperatures, e.g. at temperatures not substantially higher and preferably lower than 577° C. Flux materials heretofore commercially employed in brazing aluminum have commonly been mixtures of predominantly chloride salts, with minor additives of fluoride in some cases. An example of a suitable flux for brazing aluminum is a potassium fluoroaluminate sold under the trade mark NOCOLOK®. Although fluxless brazing procedures have been devised, their use is limited because of economic and other considerations arising from the special conditions and equipment required for successful practice of such procedures.
Flux-coated particles of braze material have been developed as an alternative to pre-cladding. The flux-coated particles provide excellent distribution of flux with the braze material, thereby maximizing effectiveness of the flux while also shielding the braze material from oxidation. The flux-coated particles are generally formed by spray forming using flux in particulate form. The flux particles contact atomized braze material droplets and melt to form a flux coating or partial flux coating on the braze material droplets, solidifying as coated powder and therefore providing a relative intimate mixture of braze material and flux.
The flux-coated particles are typically condensed and gathered on a cooled support pillar to form a pillar block of the flux-coated particles. The pillar block may be employed in later brazing applications in the as-formed form. By-product flux-coated particles that are not gathered by the cooled support pillar are often recycled. While alternative delivery vehicles for the flux-coated particles have been proposed, such alternative delivery vehicles present a host of difficulties. For example, it has been proposed to mix the flux-coated particles with liquid resin or binder or, alternatively, to dust the flux-coated particles onto the liquid resin after the liquid resin has been applied to a desired surface. The liquid resin is then cured to form a cured resin and to ensure adhesion of the flux-coated particles to the desired surface. However, use of the cured resin may present handling and application difficulties. For example, the cured resin must generally be removed, e.g., by pre-heating the braze-coated surface to decompose/pyrolyze the cured resin. Resin decomposition products can react with both the flux and substrate, thus inhibiting the brazing process. Further, adhesion of the flux-coated particles to the surface to be brazed restricts effective dispersal and surface coverage of the flux-coated particles on the surface to be brazed. It has also been suggested to include the flux-coated particles in an aqueous slurry. However, with use of both liquid resin and aqueous slurry as delivery vehicles for the flux-coated particles, particle segregation is a concern. Once segregated, the flux-coated particles are difficult to effectively re-disperse owing to generally high viscosity of the compositions. Of course, particle segregation severely impacts consistent quality of braze fillets formed using the flux-coated particles. Furthermore, the use of a liquid resin and flux coated particle slurry that is susceptible to segregation cannot be used to apply such particles in a large volume to a small surface area.
Accordingly, it is desirable to provide flowable brazing compositions and methods of brazing metal articles together using the flowable brazing compositions that employ flux-coated particles and that resist particle separation. In addition, it is desirable to provide flowable brazing compositions and methods of brazing metal articles together using the flowable brazing compositions that enable effective surface wetting and flow of flux-coated particles across a surface to be brazed. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.